1. Field of the Invention
The invention relates to a system and a method of controlling transmission electric power in a base station used in a CDMA (Code Division Multiple Access) system.
2. Description of the Related Art
In the past, a CDMA system for cellular mobile communication has been controlled based on the standard IS-95-A of TIA/EAI (Telecommunication on Industry Association/Electronic Industry Association).
In the CDMA system, since a plurality of mobile stations use the same frequency band in common, it is necessary to decrease an interference of electric power between the mobile stations and to increase the number of the mobile stations, which enable multiple access.
To this end, transmission electric power must be accurately controlled in each base station to a proper value. In this case, the proper value of the transmission electric power is usually selected such that a data error rate falls within an allowable range at a minimum value of the transmission electric power when a base station receives a signal from a mobile station. That is, the electric power received by the base station or desired-to-undesired signal ratio (SIR) determined from the received electric power is uniformly controlled regardless of a location of the mobile station.
Also, the mobile station which is busy may communicate with a plurality of base stations through the same frequency band without any interruption of communication by repeating soft handoff (or softer hand handoff) to smoothly change the base stations from one to another. Herein, the mobile station always receives pilot signals from a base station which currently communicates with and the neighboring base stations, in order to add a new base station as an object of soft handoff or delete the base station as an object of soft handoff. Then, from electric field strength of the received pilot signal, received desired signal energy-to-whole received energy ratio (Ec/Io ratio) is measured, and the measured ratio is compared with a threshold value which is given as a reference value which is determined in relation to addition of an object of soft handoff or which is determined in relation to deletion of the object of soft handoff. For example, when an Ec/Io ratio of pilot signal from a base station exceeds a threshold value T-ADD, soft handoff addition procedure is performed.
Herein, it often happens that a base station stops transmission, for example, to maintain or to check over a CDMA communication system. In such a case, since the pilot signal from the base station is rapidly stopped, a mobile station which communicates in a service area with the stopped base station and the neighboring base stations must quickly perform the soft handoff addition procedure for the neighboring base stations.
In the procedure, it is required to transmit messages between the mobile station and the base station with which the mobile station communicates.
Techniques related to soft handoff in the CDMA communication system are disclosed, for example, in Japanese Laid-Open Publications No. H09-326754 (namely, 326754/1997) and No. H10-145834 (namely, 145834/1998).
However, no consideration is made at all in the above-mentioned techniques about the fact that each base station is interrupted or stopped for maintaining the base station and about the problem resulting from such an interruption of each base station.
In fact, when a base station which is communicating with mobile stations stops transmission suddenly, a call disconnection inevitably takes place during communication in a radio link or channel between the base station and the mobile stations which are communicating in a service area with the base station.
Therefore, it is an object of the invention to provide a device and a method of preventing occurrence of disconnection of a radio network between mobile stations and base stations (a base station and the neighboring base stations) when the base station stops transmission in maintaining of itself.
To prevent the disconnection, the method of the invention reduces transmission electric power of the base station gradually in stopping transmission of the base station due to its maintenance, and performs soft handoff deletion procedure for the base station to be stopped and soft handoff addition procedure for a new base station, until transmission is completely stopped. Namely, a duration time from a start of the reduction of the electric power to a stop of transmission is desirably controlled less than a period of execution time of the soft handoff addition procedure and the soft handoff deletion procedure.
According to a first embodiment of the invention, a method of controlling transmission electric power in a first base station communicating with a mobile station is provided. The method comprises the steps of gradually reducing the transmission electric power and consequently stopping transmission in the first base station, and performing soft handoff deletion procedure for the first base station and soft handoff addition procedure for second base stations without disconnection between the mobile station and selected one of the first base station and the second base stations.
According to a second embodiment of the invention, a control system which controls transmission electric power is provided. The system comprises a first base station which gradually reduces transmission electric power of itself and consequently stops transmission, and a mobile station which communicates with the first base station and performs soft handoff deletion procedure for the first base station and soft handoff addition procedure for second base stations without disconnection between the mobile station and selected one of the first base station and the second base stations.
According to a third embodiment of the invention, a base station comprises a control unit which controls to gradually reduce transmission electric power of itself and consequently stop transmission, and a sending unit which sends a signal based on electric power which is controlled by the control unit.
According to a fourth embodiment of the invention, a computer readable medium which stores a program operable for controlling transmission electric power in a first base station communicating with a mobile station is provided. The program comprises the steps of gradually reducing transmission electric power and consequently stopping transmission in the first base station, and performing soft handoff deletion procedure for the first base station and soft handoff addition procedure for second base stations without disconnection of radio networks between the mobile station and selected one of the first base station and the second base stations.